The invention relates to a composite body comprising a first part of ceramic material, to a second part of metallic material, of ceramic material or of a mixture of metallic and ceramic material, and a third part of a melting ceramic which seals together the first and second parts, and comprises a frit of Al.sub.2 O.sub.3 and either one or more alkaline earth metal oxides or one or more rare earth metal oxides or a combination of these two oxides.
The term "ceramic material" is to be understood to mean in this description and the appended claims a material of crystalline oxidic substance which contains at least alumina. This may be, for example, monocrystalline sapphire. Another possibility is easily sintered polycrystalline alumina or yttrium aluminate garnet.
These materials, which are suitable to be used as a wall material for discharge vessels of high-pressure discharge lamps, can be heated for a long time at temperatures up to about 1300.degree. C. and have at such high temperatures a high resistance to attack by alkali metals and halides. Heating for a long time above 1300.degree. C. generally leads to a considerable evaporation of the relevant materials. Heating for a short time to 1750.degree. C. to 1800.degree. C. is possible, however, without unfavourable consequences. However, heating above 1750.degree. C. to 1800.degree. C., even for a short time, leads to the aforementioned evaporation to a very great extent and even to spontaneous formation of ruptures.
The term "rare earth metal oxides" is meant include the following oxides: Sc.sub.2 O.sub.3, Y.sub.2 O.sub.3, La.sub.2 O.sub.3, Ce.sub.2 O.sub.3, Pr.sub.2 O.sub.3, Nd.sub.2 O.sub.3, Sm.sub.2 O.sub.3, Eu.sub.2 O.sub.3, Gd.sub.2 O.sub.3, Tb.sub.2 O.sub.3, Dy.sub.2 O.sub.3, Ho.sub.2 O.sub.3, Er.sub.2 O.sub.3, Tm.sub.2 O.sub.3, Yb.sub.2 O.sub.3, Lu.sub.2 O.sub.3.
A "melting ceramic" means in this description and the appended claims a sealing material having a lower softening temperature than the parts of the composite body to be sealed, which may be present in the virteous phase and in the crystalline phase as well as in a combination of these two phases.
A composite body of the kind mentioned in the opening paragraph is known from European Patent Application No. 0060582 A1 (PUN 9968 EU). Such a composite body may be a high-pressure discharge lamp provided with a discharge vessel having a wall of ceramic material, through which a current lead-through member is passed, which is sealed thereto in a gas-tight manner by means of the melting ceramic. Frequently used high-pressure discharge lamps are high-pressure sodium lamps and high-pressure metal halide lamps. In lamps of both types, in the operating condition the wall of the discharge vessel is subjected to a high temperature and the filling of the discharge vessel is ionized in part. The melting ceramic used in such lamps therefore should be resistant to attack by sodium and metal halides at high temperatures.
The gas-tight seal, which provides a high resistance to attack by sodium and halides, can be obtained by means of the known meltin ceramic. In general a gas-tight seal is obtained by a finely crystalline melting ceramic. It should be noted that in general the thermal coefficients of expansion of the sealed parts are different so that stresses can occur in the melting ceramic. In case of a finely crystalline material, the possibility of crack formation is smaller than in case of coarsely crystalline melting ceramic. The term "crystalline" is to be understood herein to mean that 80% of the crystals have dimensions of 10 .mu.m or more. A melting ceramic is considered to be finely crystalline when the dimensions of 80% of its crystals lie between 1 and 5 .mu.m.
A further advantage of a finely crystalline melting ceramic is that during sealing the possibility of occlusions and micro-cracks is considerably smaller than in the case of coarsely crystalline melting ceramic.
It has been found that the known melting ceramic, although it is finely crystalline for the major part, nevertheless under given circumstances forms large crystals (&gt;20 .mu.m) and moreover can comprise bubble-shaped cavities. The bubble-shaped cavities ("bubbles") are due to gas desorption during the processing of the melting ceramic. The presence of large crystals is in practice always associated with micro-cavities. Micro-cavities and bubbles are detrimental to the strength and resistance of the melting ceramic.
Another problem arising in practice is that during the sealing-in process .beta. aluminate crystal structures can be formed. The probability of .beta. aluminate crystal structures being formed is found to increase considerably at higher values of the maximum temperature during the sealing-in process.
.beta. aluminate crystal structures are very sensitive to attack by especially by sodium and are therefore very undesirable in a lamp having as a filling constituent sodium or sodium halide.
The invention has for its object to provide a means by which the seal between the parts of the composite body remains substantially free from both large crystals and .beta. aluminate crystal structures whilst maintaining the favourable properties of the melting ceramic.